1. Field of the invention:
This invention relates to a semiconductor laser device that suppresses the generation of heat on at least one light-emitting facet.
2. Description of the prior art:
Conventional semiconductor laser devices are classified into two groups, one of which is of a gain guided-type and the other of which is of an index guided-type, based on the optical waveguiding mechanism. The index guided semiconductor laser devices are more useful from the practical viewpoint, because of the stability of the transverse mode that is important in practical use. A number of structures for index guided semiconductor laser devices have been developed. Especially, inner stripe semiconductor laser devices with a channeled substrate have been put into practical use because of the ease of their manufacture and their high yield. As examples of this kind of structure, there are VSIS (V-channeled substrate inner stripe) semiconductor laser devices, CSP (channeled substrate planar) semiconductor laser devices, and BTRS (buried twin ridge substrate) semiconductor laser devices.
FIG. 7A shows a VSIS semiconductor laser device, which is produced as follows: On a p-GaAs substrate 10, an n-GaAs current blocking layer 22 is formed. Then, a V-channel 3 is formed in a manner to reach the substrate 10 through the current blocking layer 22, resulting in a current path. Then, on the current blocking layer 22 including the channel 3, a p-Ga.sub.1-y Al.sub.y As cladding layer 11, a flat p-Ga.sub.1-x Al.sub.x As active layer 12, and an n-Ga.sub.l-y Al.sub.y As cladding layer 13 are successively formed. Then, on the cladding layer 13, an n-GaAs cap layer 14 for obtaining an ohmic contact is formed. The channel 3 functions to confine current therein and also creates an optical waveguide defined by the channel width within the active layer. Thus, in this VSIS semiconductor laser device, light of the active layer 12 on the outside of the channel 3 is absorbed by the n-GaAs current blocking layer 22, so that an effective refraction index distribution can be formed within the active layer, resulting in an indexguided structure (i.e., an optical waveguide) within the active layer 12. The said optical waveguide suppresses the high-order transverse mode gain, even with a channel width as wide as 4-7 .mu.m, and laser oscillation can be attained in the fundamental transverse mode even with high output power.
FIG. 7B shows a CSP semiconductor laser device, which is produced as follows: A U-shaped channel 4 is formed in an n-GaAs substrate 30, after which an n-Ga.sub.1-x Al.sub.x As cladding layer 31, a flat Ga.sub.1-x Al.sub.x As active layer 32, and a p-Ga.sub.1-y Al.sub.y As cladding layer 33 are successively formed on the substrate 30 including the channel 4. Then, an n-GaAs cap layer 34 is formed on the cladding layer 33. A p-diffusion region 35 is then formed over the channel 4 from the top of the cap layer 34 to the middle of the cladding layer 33, resulting in a current path. The channel 4 creates an optical waveguide defined by the channel width within the active layer 32. That is, an index guided structure is formed within the active layer 32 based on an effective refraction index distribution formed in the active layer 32. Because the active layer 32 is close to the n-GaAs substrate 30 at the outside of the channel 4, the light of the active layer 32 is absorbed by the n-GaAs substrate 30, and even if the channel width is as wide as 4-7.mu.m, the gain of a high-order transverse mode is suppressed, and this CSP semiconductor laser device can oscillate the laser in the fundamental transverse mode up to a high output power.
FIG. 7C shows a BTRS semiconductor laser device, which is produced as follows: On a p-GaAs substrate 60 with a ridge 60a formed in the resonating direction in the center thereof, an n-GaAs current blocking layer 65 is formed. Then, a U-shaped channel 67 is formed in the n-GaAs current blocking layer 65 in a manner to reach the ridge 60a of the p-GaAs substrate 60 through the current blocking layer 65. Then, on the current blocking layer 65 including the channel 67, a p-Ga.sub.1-y Al.sub.y As cladding layer 61, a Ga.sub.1-x Al.sub.x As active layer 62, an n-Ga.sub.1-y Al.sub.y As cladding layer 63, and an n-GaAs contact layer 64 are successively formed. The channel 67 of this BTRS laser device creates an optical waveguide (i.e., an index guided structure based on the effective refraction index distribution) defined by the channel width within the active layer 62, as well. The light of the active layer 62 is absorbed by the n-GaAs current blocking layer 65 at the outside of the channel 67, and this BTRS laser device can oscillate a laser in the fundamental transverse mode even with a high output power.
However, with these conventional semiconductor laser devices with inner stripes, because light is absorbed at both edges of the channel, even at times of high output power operation, there is an increase in the amount of light that is absorbed in the direction of the substrate with an increase in the amount of light resonating in the optical waveguide of the active layer that functions as a resonator, and because of that increase in the amount of heat absorbance, heat generated at both edges of the channel in the vicinity of the light-emitting facets becomes large. For example, the temperature of both edges thereof in the vicinity of the facets becomes about 200.degree.C. at a 30 mW operation. With conventional semiconductor laser devices, the optical power density becomes the greatest at the light-emitting facets, and accordingly an increase in the temperature of the laser devices becomes the greatest at the light-emitting facets, causing deterioration of the laser devices at the said light-emitting facets, which limits the lifespan of the laser devices.